Motor vehicles, and in particular trucks, are generally provided with an air conditioning system, hereinafter called “main”, which comprises a compressor, a condenser, an expansion valve and an evaporator defining in a known way, a main refrigerating circuit in which a refrigerant liquid flows. The compressor is driven by the traction engine of the vehicle; therefore the engine must be kept running in order to operate the compressor also when parked. This is evidently a not very efficient engine use, with consequent excessive consumption and emission of polluting agents.
In order to solve the aforesaid problem, it has been proposed to use an auxiliary air conditioning system comprising an auxiliary thermal engine, of reduced power with respect to the traction engine, and an auxiliary refrigerating circuit comprising in turn an auxiliary compressor, condenser, expansion valve and evaporator. In this way, when parked, it is sufficient to keep the auxiliary engine running, which has reduced consumption with respect to the main traction engine, but which can also work in maximum efficiency conditions, thus minimising consumption and polluting emissions.
An auxiliary system of this type is disclosed in WO-A-05108133. This system is fully autonomous with respect to the main system and may work according to a refrigerating cycle to extract heat from the passenger compartment, and according to a reversed refrigerating cycle, i.e. as a heat pump, to warm the passenger compartment. The system conveniently comprises a heat exchanger installed inside the passenger compartment (hereinafter called “internal exchanger”), having the function of an evaporator in refrigerating cycle operation and of a condenser in heat pump operation, and an integrated external unit easily installable on the chassis of the vehicle, which groups together all the remaining refrigerating circuit components. In particular, such external unit comprises a second heat exchanger (hereinafter called “external exchanger”) working as a condenser in refrigerating cycle operation and as an evaporator in heat pump operation.
In this operating mode, the drawback of ice forming on the external exchange occurs in the event of outside temperatures close to 0° C. To solve the aforesaid problem, the use of an electrical heater associated to the external exchanger has been proposed. This entails additional costs, more complex system and controls and a lower energy efficiency.
Another problem, typical of heat pumps, consists in the fact that as the outside temperature decreases so does the operative fluid temperature and therefore the thermal power that the internal exchanger can output into the passenger compartment.